In various sectors, such as industry, medicine and motor vehicles, use is known of piezoresistive pressure sensors incorporated in a transducer for measuring a pressure of a fluid.
A flat piezoresistive pressure sensor generally comprises a rigid flat support made of a ceramic material, a flat flexible membrane made of a ceramic material having a flat external face exposed to a pressure of a fluid and a flat internal face delimiting, in cooperation with a flat internal face of the support, a chamber accommodating a deformation of the membrane, a resistive Wheatstone bridge applied on the flat internal face of the membrane for detecting the deformation thereof.
One of the critical parameters of a pressure sensor is the offset, which must fall within a precisely-specified acceptance range.
By offset is meant the output signal of the resistive bridge when the membrane is subjected to a reference pressure.
The offset is prevalently caused by the tolerance in the construction of the resistive bridge using the silk screen printing method.
To correct the offset and reduce as far as possible the value, the resistive bridge is provided with a calibration resistor applied on the external face of the support and connected to the rest of the resistive bridge by suitable through-holes through the thickness of the support wall, which through-holes are clad with electrically-conductive material.
The calibration resistor in particular is inserted on the support in parallel or in series with a branch of the bridge and is cut with the laser for the appropriate choice of the value.
Owing to the presence of the calibration resistor, the pressure sensor has a production process that is very complex and consequently expensive, especially for the carrying out of the electrical connection between the calibration resistor and the rest of the resistive bridge.
Moreover the calibrating of the pressure sensor is rather complicated as it requires a delicate choice of the point of the calibration resistor at which to carry out a precise and accurate laser cut.
Further, the need to calibrate the pressure sensor after assembly thereof means that the calibration resistor must remain in an accessible position and therefore also exposed to the risk of damage.